This invention relates to a double-walled vacuum insulated container for product storage at cryogenic temperatures, and a method for manufacturing same.
Double-walled vacuum insulated containers are widely used for the long-term preservation of living tissue, sperm and whole blood and for storage and transportation of valuable cryogenic liquids. These containers usually employ in the vacuum insulation space, a composite multi-layered, external load-free insulation comprising low conductive fibrous sheet material layers composed of fibers for reducing heat transfer by gaseous conduction, and thin flexible sheet radiation barrier layers. The radiation barrier layers are supportably carried in superimposed relation by the fibrous sheet layers to provide a large number of radiation barrier layers in a limited space for reducing the transmission of radiant heat across the vacuum space without perceptively increasing the heat transmission by solid conduction thereacross. Each radiation barrier layer is disposed in contiguous relation on opposite sides with a layer of the fibrous sheet material, the fibers being oriented substantially parallel to the radiation barrier layers and substantially perpendicular to the direction of heat inleak across the insulating space.
One such commonly used fibrous sheet material described in Matsch U.S. Pat. No. 3,009,600 is small diameter glass fibers (about 0.5 microns diameter) in permanently precompacted sheets of about 1.5 mils thick and weighing about 1.5 gms. per sq. ft. (hereinafter referred to as "glass microfiber"). The extreme fineness of glass microfiber affords mechanical integrity of the separator in very thin sheet form without reliance on chemical binders to "glue" the fibers together. When an insulation comprising glass microfiber sheets alternating with thin aluminum foils is installed at near-optimum density of 70 layers/inch and in a vacuum of less than 0.1 micron Hg absolute, its thermal conductivity is about 2.5.times.10.sup.-5 Btu/hr.ft..degree. F. If a 29-liter liquid nitrogen container is provided with such an insulation, it is capable of obtaining a normal evaporation rate (NER) of about 0.33 lbs. of the liquid nitrogen per day.
The disadvantages of glass microfiber are its high cost and its extreme sensitivity to mechanical compression. The latter characteristic has been explained as the result of increasing the number of fiber-to-fiber contacts within the sheet which in effect shortens the path of heat flow between the reflective foils separated by the sheet. In practical usage of thin permanently precompacted-form spacers in multiple layer insulation, it is usually impossible to avoid high compression at least in localized areas of the insulation.
An alternative glass fiber material, described in Clapsadle U.S. Pat. No. 3,145,515, is large diameter (1.6-2.6 micron) fibers in fluffy uncompacted "web" sheets without significant binder. Lack of strength and poor handleability, characteristics of this separator, are accommodated by supporting the fiber sheet continuously on another, stronger sheet material such as the reflective foil used in the insulation. Thus, the supporting foil may be interleaved with the delicate fiber sheet at the time the latter is produced, and thereafter, the two components are handled and applied together during vessel manufacture as a single composite layer. The resultant multi-layer insulation is excellent for large vessels requiring moderately effective insulation, but its thermal conductivity (about 10.times.10.sup.-5 Btu/hr.ft..degree. F.) does not meet the extreme requirements for small cryogenic containers with long "holding" time.
An alternative to glass fiber sheets are the organic fiber separators described in Gibbon et al U.S. Pat. No. 3,265,236 having certain specifications including much lower intrinsic thermal conductivity than glass. By way of example, the patent states that with a rayon fiber, a minimum thermal conductivity for multi-layer insulation is obtained which is equal to glass fiber multi-layer insulation, but with fiber 16 to 24 times larger in diameter. In order to obtain strength and good handling characteristics with large fibers in thin sheets, the patent contemplates the use of binders such as polyvinylacetate in quantity such as 14 wt.% of the sheet. Sheet materials weighing 1.475 and 1.01 gms/ft..sup.2 are disclosed. In addition to rayon, other disclosed suitable organic fiber materials are cotton, Dacron, Dynel and nylon. Dacron is a polyester produced by condensation of dimethylterephthalate, nylon is a polyamide and Dynel is a copolymer of vinyl chloride-acrylonitrile.
According to the Gibbon et al patent, fiber sheets may be produced from these organic materials using either paper-making or textile machinery. Textile sheets have not been used in commercial installations, however, due to relatively high cost and poor thermal efficiency. In paper-making machinery, the fibers are laid down on a moving screen and are compressed while wet as between rolls, so that after drying, the paper retains a compressed condition. Sheet materials produced of large diameter rayon fibers (e.g., 12-18 microns), in low thicknesses (e.g., 1-2 mils) and in light weight (e.g., 0.8 to 1.5 gms/ft..sup.2) afford excellent separators for composite insulations. One such material applied at near-optimum layer density of about 70 layers per inch provides a thermal conductivity on the order of 2.times.10.sup.-5 Btu/hr.ft..degree. F. The fiber sheets are reasonable in cost, being readily produced on wet-process, paper-making machinery.
Unfortunately, use of certain of the aforementioned organic fibers presents other problems. The rayon fiber and many other organic fibers have a strong affinity for water. When exposed to atmosphere of normal humidity, such fibers absorb large amounts of water, amounts between 8 to 20% of the fiber weight being typical. When these are evacuated, the absorbed water is evolved profusely over a wide pressure range and over extended periods of time.
When multiple-layer insulations are installed between the walls of a cryogenic vessel and evacuated, suitable provision must be made in the insulation space for immobilizing gases which evolve from materials exposed to the vacuum and which inadvertently enter the space through minute leaks. The usual means for scavenging these gases is a highly active adsorbent such as molecular sieve 5A (calcium zeolite A) which is installed against the cold outer wall of the inner vessel as disclosed in Loveday U.S. Pat. No. 2,900,800. The absorbent, when chilled to liquid nitrogen temperature, has a very high affinity for most atmospheric gases. Its capacity for water is even higher and since pre-adsorption of water reduces its capacity for oxygen, nitrogen and argon, it must be installed in a pre-dried condition and exposed to normal humid atmosphere for minimum time prior to sealing and evacuating the insulation space. When thus installed, a relatively small quantity of molecular sieve 5A is capable of maintaining absolute pressure below 0.5 micron Hg during cold service conditions. Hydrogen is also evolved in vacuum insulation spaces and is not readily immobilized by physical adsorbents. However, hydrogen gas can be removed by reaction on a suitable getter such as palladium oxide as disclosed in Matsch et al U.S. Pat. No. 3,108,706.
Whereas satisfactory double-walled cryogenic storage containers can be produced from materials and by procedures outlined above, the evacuation of such containers in expensive and time consuming. This final step in the manufacturing procedure becomes a serious bottleneck in production, requires a large number of evacuation stations in a factory, consumes substantial amounts of energy, and increases significantly the production costs of the containers. The time required for evacuation usually far exceeds 8 hours so that this terminal step extends over into one or more days processing time. In this procedure the space between the inner vessel and the outer casing is evacuated for an external period of time to remove not only the free gases from the space but also adsorbed gases from the huge surface areas of the shields, fibers, walls and absorbent within the insulation space. Initial evacuation by a mechanical pump to a pressure in the insulation space on the order of 1000 microns Hg proceeds quickly, usually in less than 15 minutes time. Then evacuation is switched to a diffusion pump for a much longer period of time to remove the slowly desorbing gases from the insulation space. The final pressure will typically fall below 50 microns Hg.
When the vessel is placed in service and the adsorbent is chilled to cryogenic temperature, the adsorbent "captures" and immobilizes remaining gas within the space and produces the desired low absolute pressure below 0.5 micron Hg. and preferably below 0.1 micron. However, a quantity of adsorbent which can be reasonably accommodated in the vacuum space will not be able to produce this low pressure or maintain such pressure for extended service life unless the preadsorbed gases are effectively pumped away from the insulation system during manufacture.
A typical evacuation pressure-time curve for a prior art container is shown in FIG. 1 as the next-to-highest curve A. The container is 29-liter capacity and is provided with rayon fiber sheet-aluminum foil multi-layer insulation and molecular sieve 5A adsorbent. As stated above, the pressure drops rapidly to a level between 100 and 1000 microns but then decreases slowly. Experience in commercial production has shown that a minimum evacuation period of 18 hours and preferably 24 hours is required to obtain satisfactory insulation performance with this system. If such extended evacuation is not performed, the necessary service vacuum of less than 0.5 micron Hg will not be obtained or maintained in the insulation.
One object of this invention is to provide an improved multi-layered thermal insulation system for the vacuum space of double-walled cryogenic storage containers, characterized by low heat conductance, low material costs, and which is easily and quickly evacuated during production prior to usage.
Another object of the invention is to provide an improved method for manufacturing a double-walled cryogenic storage container employing organic fiber type multi-layered thermal insulation which does not require predrying of the fibrous sheet, a dry adsorbent and assembly in a dehumidified atmosphere.
Other objects will be apparent from the ensuing disclosure and appended claims.